Optical attenuating elements can be manufactured by welding two optical fibers to each other with a lateral offset of the fibers, i.e. a splice having an intentionally produced lacking alignment of the cores of the fibers is manufactured and thus having a large loss. Then a welding device of the automatic type having a modified control program can be used. The control of the welding process can be performed in real time. The electronic processor of the welding device cans, for example, in real time get information from a power meter measuring the power of light coming from a light source and propagating through the splice during the welding process, and use the information to control the electric arc. The method comprises first selecting a desired lost. Then a splice having an offset is made. During the heating in the splicing process a current loss is essentially continually read. The molten glass material in the fibers has a surface tension reducing the offset and the loss gradually falls during the heating. When the loss has decreased to the desired loss the electric arc is stopped and thereby the heating is stopped.
This method is for example described in the published International Patent Application No. WO 95/24665 corresponding to U.S. Pat. No. 5,638,476, in U.S. Pat. No. 5,897,803 and in the published European Patent Application No. 0594996.
It appears that several problems exist in this method. The main problem is however that the splice loss in the resulting splice does not become correct when using the method. It is thus a basic problem that the loss determined in the splice during the welding process according to the method differs from the loss that is measured directly after finishing the welding process. Most often the loss is lower after the end. The difference is about 0.5-2 dB for losses of about 3-15 dB for a reference point of about 200 μW, i.e. an input light power of approximately this value.
This effect could be explained by the fact that more light hits the detector which has a broad spectral responsiveness due to the fact that the fiber glows or that light from the electric arc is transmitted in the fiber. However, from tests when the light source is inactivated it has been possible to find that the light emitted by the fibers and the electric arc contributes very little. The power is in the magnitude of order of nW which corresponds to a very small part of a measured difference of 0.5-2 dB in the case where the reference point is about 200 μW.
The explanation of the difference is more probably associated with the fact that the optical character of the splice is changed due to the large heat differences that exist. E.g., the refractive index could be changed, this resulting in changed conditions for total reflection or in changes of the mode field diameters on which the loss depends. The steps could also be thought of as being caused by a difference in lateral offset between the fibers depending on whether the splice is hot or cold.